Research Sheds Light on Challenges of Interpreting Brain Activity

Correlation doesn't necessarily mean causation. It's a warning that echoes throughout the halls of science, but is not always heeded. A new study in the journal Nature by associate professor Alex Huk and graduate students Leor Katz and Jacob Yates provides a perfect case study.

For years, neuroscientists have noted that when animal subjects watch moving dots on a screen and give a sign that indicates they know which way the dots are moving, neurons in a part of the brain called the lateral intraparietal (LIP) light up. Neuroscientists often study the processing in the brain that takes visual stimuli and turns them into reports about perception as a simple form of what they call "decision-making." In other words, the subject has made a decision that a particular set of dots, for example, were moving to the left.

Not only do LIP neurons fire during this task, but the way they fire predicts the subject's choices before they make it. And now you—like many neuroscientists before you— might begin to think that these brain cells must play a critical role in decision-making.

Only that turns out not to be the case. The new research acts as "kind of a public service announcement," for people who would jump to such a conclusion, Huk notes.

He and the other University of Texas at Austin researchers used a chemical to reversibly inactivate the neurons in the LIP, and found that subjects apparently were just as able to successfully complete the task as before: their ability to make decisions about the motion they were viewing was not impaired. So, the researchers concluded, the LIP doesn't play a critical role in decision-making, despite sending all the obvious signals that it might.

"This result is important because it underscores the important distinction between correlation and causation," Katz says. "Just because the neural activity in area X is correlated with behavior Y, does not guarantee that area X is solely causing behavior Y."

Many brain areas besides LIP have activity that correlates with decisions, but in practice, says Huk, many researchers, himself included, have focused on LIP.

"Our study suggests that if one wants to understand how the brain makes these simple decisions, it's going to be really helpful to put more focus on the larger network," he says. "And if we want to figure out what LIP is critical for, we're going to need to think about other sorts of tasks and mental functions.

To understand what strong versus weak motion means for this particular visual task, it helps to see what the subjects see. The following videos are similar, but not identical, to the ones used in this study. The video with 100 percent coherence represents strong motion to the right because it's very obvious that all the dots are moving to the right. Near the other end of the spectrum, a video with 5 percent coherence means that the dots are overall moving to the right, but in a very subtle way that can be hard for a subject to accurately report.

This research was supported by the Howard Hughes Medical Institute, the McKnight Foundation, the National Eye Institute and the National Institute on Drug Abuse.